Its the record at the Land Registry that counts. What has probably happened is your bank (mortgage company) has realised it is holding onto a redundant piece of paper in its vaults and returned it to you.
To sell the garage you are doing to have to separate the title plan of your land, which will be in two little red rectangles on your title plan to reflect that you own two parcels of land under the same title number. Then you can sell it. Possible to DIY, but probably best for you to ask a solicitor, who you are going to have to employ to transact a sale anyway.
Are you really sure it is a good idea to do this? Depending where you live, selling the garage may seriously reduce the value of your house - by far more than the price to get for the garage.